Batavia Quarry Beach leak analysis: It's the river

When you jump around in the water at Harold Hall Quarry Beach in Batavia, your feet don't land on the bottom of the former quarry.

Your toes are on 1 to 2 feet of beach sand. Below that is a fabric membrane, below that there is supposed to be a 4-inch clay liner, below that is 5 to 16 feet of fill, and below that, finally, is the fractured dolomitic limestone bedrock at the bottom of the hole.

And because the nearby Fox River is now 3 feet lower than that bedrock bottom, pool water leaks through all of that, an engineering firm confirms in a report unveiled by the Batavia park board Tuesday.

AECOM of Vernon Hills has also recommended a way to leak less: Install a new geomembrane liner system in the swimming area.

And in the diving well, where the membrane isn't practical and the rock bottom is part of the aesthetic appeal, use a combination of shotcrete, rock grout and dental cement to plug fractures in the limestone.

The board directed the staff to investigate the treatments for the diving well and further evaluate the site. It also ordered the report to be sent to the Illinois Department of Public Health, to determine if the recommendations would constitute any change in their classification of the facility as a beach, according to executive director Mike Clark.

The 66-page report notes that a clay liner was installed in the swimming area during renovations in the early 1990s. But that liner has degraded, with clay entering the fill, so that it is only about 2 inches thick now. And, the engineers reported, the fabric membrane is usually placed underneath the clay, not on top.

Installing geomembrane liner system in the swimming area would cost $6 to $7 per square foot, the firm estimates.

That does not include recommended add-ons such as a geosynthetic fabric on top of the liner and a sand-filled cell system to protect the liner and fabric from ultraviolet rays, swimmers' feet and maintenance equipment. It did not provide a square-foot cost for the diving area. Overall, Quarry Beach pool is 60,000 square feet; the swimming area alone is about 43,000 square feet.

The current clay liner was supposed to keep water in. But that was when the water level of pool was several feet higher than that of the nearby Fox River.

This report confirms what park district employees have suspected for several years: the 18-inch drop in the river level after the dam removal changed hydrostatic pressures in the limestone. Water has always leaked into the limestone fractures, but had nowhere to go when river levels were higher.

The district has estimated that during the swim season, the quarry loses 250,000 gallons of water a day, way more than could be accounted for by evaporation.

It bases its calculations on the amount of water it is pumping from a deep well it added in 2006 to the site, and observations that when the river rises, it has to pump less water.

The report outlines several approaches that could be taken for the swimming and diving areas, including using shotcrete on the swimming floor and using a combination of shotcrete, dental cement and rock grout in the diving area.